Thou (unit)

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The thou (abbreviation th) or mil (Chiefly United States) is a unit of length in the Imperial System and the United States Customary Units. It is equal to ​11,000 of an inch.

Etymology[change | change source]

Thou comes from the Germanic Languages, a shortening of thousandth.

Mil comes from the Latin Languages, a shortening of mille.

History[change | change source]

The thou was introduced in 1844 by Joseph Whitworth as a substitute to traditional fractions such as ​16413211618 which cannot be defined simply in hundredths unlike ​14 (0.25) or ​12 (0.5). Traditional fractions continued to be used, as they were a substantial part of other units of length.

Length[change | change source]

International Thou Inch Millimetre
Thou 1 11,000 0.0254

Usage[change | change source]

The thou is used in engineering and manufacturing mostly.